This invention relates to an improvement of ignitability of a spark plug.
In recently developed automotive engines, lean air/fuel mixture combustion is desired for purifying exhaust gases, and an spark advance tends to increase in order to reduce the fuel consumption, particularly during idle speed. For this reason, the idle speed does not spoil engine power. The present inventor has found that when the spark advance is increased, a spark discharge is generated during an engine compression stroke. Flame nuclei produced by the spark discharge are, as shown in FIG. 1, propagated from a position A to a position A' on a center electrode D side of a spark plug C in compliance with movement of a piston B. A quench operation of the center electrode is strongly applied to the flame, and this quench operation mainly causes a misfire during idle speed and a low speed rotation of the engine. A spark plug having a groove in a center electrode or an outer electrode in order to enhance ignitability has been heretofore provided. However, since such a spark plug having a groove is made irrespective of the above-noted fact, the enhancement of the ignitability is still deficient.
Within the prior art many such spark plug designs having various cuts, holes, etc. in the electrodes are known. Typical are the annular discharge portions shown in the inner and outer electrode surfaces of U.S. Pat. No. 4,015,160, the annular ring and channel arrangement of U.S. Pat. No. 4,023,058 and the V-type ground electrode with a channel shown in U.S. Pat. No. 2,226,415. Additional prior art is shown in U.S. Pat. No. 3,970,885 which includes, in addition to various groove embodiments a projection provided on the ground electrode (element 36, FIG. 25). As set forth in that patent, the projection is disposed in the spark discharge area confronting the tip surface of the center electrode. The flame nuclei produced by the spark are rapidly spread out over the projection to facilitate growth of flame nuclei and easy propagation of flame. The hallmark of all these prior art devices is the modification of the ground electrode to limit the area of the grounded surface. However, it has been found that many deficiencies remain, in actual use, so these spark plugs do not satisfactorily perform, especially in an idle speed engine condition.